1. Field of the Invention
The present invention relates to a semiconductor bonding apparatus such as a wire bonding apparatus using both nail-head ultrasound and heat and pressure bonding, an ultrasonic wedge type wire bonding apparatus, a single TAB bonding apparatus, etc., and more particularly to an ultrasonic horn equipped at one end with a bonding tool and at another end with a vibration-generating source such as an electrostrictive strain element, a magnetostrictor, etc.
2. Prior Art
A conventional nail-head heat and pressure type wire bonding apparatus is constructed, for example, as shown in FIG. 5.
A supporting shaft 2 is fastened to a bonding arm 1. The supporting shaft 2 is supported on a bonding head (not shown) either directly or via a lifter arm so as to be rotatable. The horn support 4 of an ultrasonic horn 3 is fastened to the bonding arm 1. The ultrasonic horn 3 essentially consists of a horn main body 6 having a bonding tool 5 through which a wire (not shown) is passed and a vibrator 7 which is screwed to the horn main body 6.
The vibrator 7 is of a structure in which a vibration-generating source 8 is attached by a screw fastening method known as a "Langevin" type system. The Langevin type assembly includes a horn attachment 9 which is attached to the horn main body 6, a vibration-generating source attachment shaft 10 which has external screws on the both ends so as to be screwed to the horn attachment 9, and an insulating pipe 11 which is fitted over the vibration-generating source 8.
In this conventional ultrasonic horn 3, the vibration-generating source 8 is attached on the other side of the horn support 4 from the bonding tool 5. The vibrator 7 is adjusted to a prescribed frequency by attaching the horn attachment 9 and nut 12 to the respective ends of the vibration-generating source 8. The acoustic length of the vibrator 7 must be an integral multiple of 1/2 wavelength. In this structure, however, there is no need for elongating the acoustic length; accordingly, a vibrator that has a 1/2 wavelength is used. The free end 13 of the vibrator 7 forms a vibrational belly (where the amplitude is large), and the horn attachment portion 14 of the horn attachment 9 also forms a belly, so that attachment is facilitated.
In operation, the vibration of the vibration-generating source 8 is transmitted throughout the ultrasonic horn 3 so that a standing-wave vibration is created in the ultrasonic horn 3, thus supplying the necessary energy to the bonding tool 5.
In an unloaded state (in which no bonding is being performed), energy accumulates in a stable manner. In a skillfully crafted ultrasonic horn 3, the dimensions are such that a node is at the horn support 4. Accordingly, even with the ultrasonic horn 3 attached to the bonding arm 1, there is little movement of the horn support 4, and therefore, little loss. In such an unloaded state, the ultrasonic horn 3 acts as a tuning fork with the horn support 4 receiving vibrations synmetrically from the left and right, so that the horn support 4 does not move to the left or right. The vibration-generating source 8 is ordinarily driven by constant-current driving, etc. so that the vibrational amplitude has a prescribed value. When energy is used for bonding by the bonding tool 5, the energy balance between the vibration-generating source side and the bonding tool side is destroyed so that the vibrational node moves, and the energy required in order to establish a balance is fed in. In this way, bonding utilizing ultrasonic waves is conducted.
In the conventional ultrasonic horn 3, the horn shows rough line symmetry with respect to the axis of the horn, and the bonding tool 5 is attached to the tip end of the horn so that the bonding tool is asymmetrical with respect to the axial center. As a result, the ultrasonic horn 3 makes its expansion and contraction only in the axial direction and move the bonding tool horizontally so as to provide energy with the bonding plane. However, due to the resistance between the bonding tool 5 and the bonding surface, it is impossible to achieve control that includes components other than horizontal components when the operation of the tip end of an actual bonding tool 5 is made. In other words, bonding which depends on the load and the bonding tool 5 is performed.
Recently, with the increased density of semiconductor devices, the addition of various chemical elements to the pad has become common in order to execute high-density wiring. As a result, pad surface shapes and oxide film structures have become diverse, and it has become difficult to maintain good bonding quality. The main causes of faulty bonding would appear to be the difficulty of diffusing the pad material and wire material, and the difficulty of destroying the oxide film. The method to solve these problems is to apply the required amount of ultrasonic energy to the pad regardless of the conditions of the pad surface.
As a result of tests, the inventors discovered that with the ultrasonic horns which have symmetrical structures with respect to the axial centers, it is not possible to provide stable ultrasonic energy with the bonding surface without regard to the conditions of the bonding surface.
The inventors found the reasons for such problems. The pad surfaces form various shapes as a result of crystals of the doped material or oxides of the same, and in some cases, a structure such as a ridge-form structure, etc. is formed which makes it difficult for the ball formed at the tip end of the wire passing through the bonding tool 5 to slip. Furthermore, in the conventional ultrasonic horns 3, as described above, only the axial direction of the ultrasonic horn 3 is considered important as the direction of vibration. Accordingly, in cases where only an axial force (energy) is applied to the bonding tool 5, stable frictional movement of the ball at the tip end of the bonding tool 5 becomes difficult to achieve. Thus, a stable bonding energy is not applied, resulting in faulty bonding.